Akin to the Neckbreaker, the Backbreaker is a tunnel that is so small that it does not allow for a UEr to stand at all. Typically, a Backbreaker will be too high for hands-and-knees style crawling but too low for a Neckbreaker.
The best way to deal with a Backbreaker is to bend forward at the waist and then tip your head to the side so you can look forward and to discourage staring at your feet as you walk. Doing so will lessen the likely hood of your running head-first into a curve or falling down a slide.
Also take lots of breaks and turn back if the tunnel makes no sign of geting bigger. If they are available, stand up fully in manhole rooms to avoid getting stiff necked.
MacGyver adds: Travel through backbreakers can sometimes be made easier with a skateboard (!) or by wearing a backpack frontwards.
Although most famous in drains, backbreakers are also prevalent in utility tunnel networks. Sometimes they lead to something really interesting, but they are famous for dead-ending in the most uncurteous ways.
After hobbled crouching in this extremely hot tunnel for a long ways while carrying a full-sized tripod, I was greeted by a wall and had no choice but to retrace my steps: